This disclosure relates to unique filter constructions for filtering fluids including gaseous or liquid streams. In particular, a unique arrangement of filter components is disclosed that can provide substantially increased lifetime for a filter construction while maintaining or improving filter efficiency. Such filters typically comprise fine fiber, a porous substrate layer that can be used in combination with a variety of supports, housings, hardware and other components in a filter structure. A fluid stream passes through the filter to remove unwanted materials such as a particulate material. As the streams pass through the filter, particulates that can be in the form of liquids, solids and mixtures thereof are removed from the mobile fluid streams.
In the practice of filtration technology, a fluid stream is typically passed through a filter structure for the purpose of removing a particulate from the stream. Any filter element will be able to remove some proportion of the particulate from the stream during the lifetime of the filter. Filter efficiencies are typically defined as the proportion of the particulate, entrained in the mobile fluid phase, removed by the filter. Filter lifetime is typically considered to be related to the period of time that the pressure drop across the filter remains below a certain predetermined level to permit acceptable operating parameters for the filter and operating equipment. A filter must obtain a sufficient removal efficiency while maintaining a sufficiently low pressure drop to obtain useful performance. A high pressure drop is characteristic of poor operating efficiency for the equipment using the filter.
As is true in many technologies, substantial trade-offs arise in any embodiment of a successful technology. Very often as efficiency increases, pressure drop also increases and lifetime often is substantially reduced. For reasons that are not entirely well defined, the pressure drop across any filter can increase substantially during operations. Filtering streams containing substantial proportions of mist or fog, inorganic aerosol, organic aerosol formed from oils, fats, carbon or other sources, or mixed aqueous inorganic organic aerosol structures often results in reduced useful life. The tightly interlocked nature of efficient fine fiber layers can cause increased pressure drop across the fine fiber layer will substantially and rapidly increase when contacted with such a fluid particulate stream. While these filters are excellent in initial operation, the filter lifetime, not efficiency is often a problem. The filters are adequate for the task but must be replaced. In view of the rapidity that such structures can increase in pressure drop, i.e. have substantially reduced service lifetimes, improvements to such filters are needed.
As is typical in any technological application, the improvement of both filter efficiency and lifetime is a long sought goal for filter manufacturers. In view of this, substantial need exists in the art for filter technology and structures that can obtain increased filter lifetime while maintaining or improving filter efficiency.
We have found that a substantially improved filter media, filter structure, and filtration process can be obtained by using a filter media having a controlled amount of fine fiber in two or more layers in a media substrate or structure. By forming the fiber in reduced amounts in two or more layers filter efficiency can be maintained or increased while lifetime can be increased. In a preferred embodiment, a first layer of fine fiber is placed on an upstream surface of the substrate, then a second layer is formed as a second surface typically downstream. The efficiency of the upstream and the downstream layers can be intentionally selected to be different. The downstream layer can have an efficiency greater than the upstream layer. The layers are placed such that the filtered fluid passes through two layers. The media can be formed into a filter structure in a variety of filter structure geometries and formats. The double sided layer of fine fiber maintains or increases the efficiency of filtration but substantially increases the lifetime of the filter. We have surprisingly found that by placing an amount of fine fiber that results in an efficiency less than about 90% in a first layer on a side of a substrate combined with one or more second layers, in a filter structure, obtains a filter having greater than 90% overall efficiency and extended lifetime. We have found that by placing an amount of fine fiber in any one layer that ranges from about 50% to less than about 90% efficiency provides these unique advantages, preferably we have found that the amount of fine fiber placed on the substrate should range from about 65% to about 85% efficiency.
We believe one mechanism by which the fine fiber layer obtains a substantially increased pressure drop results from the xe2x80x9cfilming overxe2x80x9d phenomenon. As filtered particulate materials interact with the fine fiber and become trapped in the fine fiber mesh or web, the particulates, particularly if they are low volatility liquids, can form a liquid film completely filling an opening pore or space in the fine fiber mesh. As these areas in the mesh are filled with fluid, the pressure across the filter rapidly increases. The filming over property can also result from interaction between particulates and the fine fiber but simply results from the filling of the unoccupied space within the fiber web causing pressure increase. Having a layer on the downstream side that is greater in efficiency than the upstream side, by more than 3% preferably 5% or more, increases overall efficiency but does not reduce lifetime because the upstream layer and the substrate remove entrained particulate and reduce the tendency of the downstream fine fiber to plugging. One measure of defining lifetime can indicate that the filter structure has completed its lifetime when the pressure drop across the filter increases to about 3 inches of water or more at a test condition of 10 ft/min of flowing medium.
The invention relates to polymeric compositions in the form of fine fiber such as microfibers, nanofibers, in the form of fiber webs, or fibrous mats used in a unique improved filter structure. The polymeric materials of the invention comprise a composition that has physical properties that provide improved efficiency and service lifetime in the unique filter structure. The polymeric materials of the invention are compositions that have physical properties that can also permit the polymeric material, in a variety of physical shapes or forms, to have resistance to the degradative effects of humidity, heat, air flow, chemicals and mechanical stress or impact while maintaining effective filtration during use.
In typical applications, fine fiber is placed on the substrate, the fine fiber layer comprises a fine fiber having a diameter of about 0.0001 to 5 microns, preferably about 0.0001 to about 0.5 micron, most preferably about 0.001 to about 0.3 micron formed in a layer that has a layer thickness of less than about 5 microns, preferably about 0.1 to 3 microns, often from about 0.5 to about 2 microns. Each fine fiber layer comprises an interlocking randomly oriented mesh of fibers that results in a mesh having a relatively broad distribution of pore size openings. For the purpose of this patent application, the term xe2x80x9cporexe2x80x9d refers to a passage or opening in the web through the fine fiber layer that is formed from a periphery of 2 or more fine fibers. The pore can result from the intermingling of a variety or large number of fine fibers creating or forming openings of a size that can be effective in traping particulate materials. While any fine fiber layer can have openings of a variety of sizes, the fine fiber layers of the invention have a substantial number of pores of a size that range from very small, i.e. about 0.001 to about 5 microns, but often range between about 0.5 and 3 microns for effective filtering. Preferably, in the structures of the invention, the pores are formed with openings having a open pore size less than 3 microns, often less than 1 micron in the form of an interlocking mesh having openings in which a major dimension of the opening is less than the diameter of the particle typically removed from the fluid passing through the filter. We have found that the tendency of fine fiber layers to obtain an increased pressure drop or to film over can be minimized by reducing the fine fiber coverage on opposite sides of a substrate layer. By placing a reduced amount of fine fiber on both sides of the substrate, the tendency of the fine fiber layers to have smaller easily closed pores as a result of filtering operations with liquid substances is substantially reduced. We believe this reduction results from a somewhat larger pore size in the reduced layer structure, however, the reduced increase in pressure can also result from reduced surface area of the fiber layer. In other words, for example, an improved filter structure can be manufactured by modifying a filter structure having a single fine fiber layer on one side of a substrate displaying an average efficiency of about 90% into a filter structure having two fiber layers that have an efficiency of less than 80%. While the first single layer fiber structure will permit about 10% of the particulate (90% efficient) to pass through the layer, a single efficient layer will have an enhanced tendency to experience quick pressure drop increase. By forming two layers having an efficiency of about, for example, 75%, a filter having an overall efficiency of about 87.5% can result with substantially reduced tendency to an increased pressure drop because of the reduced amount of fiber in the fine fiber layer.
The combination of two layers of fine fiber on opposite sides of a planar media layer, each layer having a reduced efficiency, provides across the entire layered structure, a substantially high efficiency. Surprisingly, this combination of two layers of fine fiber on opposite sides of a planar media layer displays an extended lifetime due to a reduced tendency to plug or film. By distributing the filter particulate throughout the layers of the construct, we believe that any undesired increase in pressure during the lifetime of the filter can be reduced since the filtered particulate does not reside in a relatively narrow portion of the fine fiber structure. We believe it is a surprise that the layered structure distributes the fine fiber throughout the layers, thus substantially improving and extending the time the filter maintains a pressure drop less than the designed maximum pressure drop. Such a distribution of fine fiber also distributes Filter constructions have obtained a variety of physical formats. Filters have been developed into planar sheet-like filter barriers, pleated panels, cylindrical or oval elements, elements formed within cylindrical cartridges, corrugated elements and others. Each of these filter formats can be constructed and arranged in a variety of known technologies. Any filter format can be used that results in the filtered media passing twice through a fine fiber layer.
The media in the filter element or structure can be treated with a fine fiber, for the purpose of this invention, the term xe2x80x9cfine fiberxe2x80x9d indicates a fiber having a fiber size or diameter of 0.0001 to 5 microns or less often 0.001 to 0.5 microns and, in some instances, substantially submicron diameters. A variety of methods can be utilized for the manufacture and application of fine fiber to the media. Kahlbaugh et al., U.S. Pat. No. 5,423,892; McLead, U.S. Pat. No. 3,878,014; Prentice, U.S. Pat. No. 3,676,242; Lohkamp et al., U.S. Pat. No. 3,841,953; and Butin et al., U.S. Pat. No. 3,849,241; all of which are incorporated by reference herein, disclose a variety of fine fiber technologies.
The conventional filter construction involves the application of fine fiber to the substrate in a single layer in a substantially complete coverage of the media. Sufficient fine fiber is typically used in the fine fiber layer such that the resulting media construction has an initial efficiency of greater than 50%, preferably greater than 80% (on an average basis) with no individual construction having an efficiency less than 30% (the efficiency test is ASTM 1215 89 using monodisperse 0.78 micron polystyrene latex particulate at 20 ft-minxe2x88x921). This efficiency test generally measures the effectiveness of the test substrate to remove from a moving air stream the recited particulate that is moving at the recited rate. The efficiency is expressed as a percent relating to the percent of the total particles tested that is removed by the filter. This test method measures the initial efficiency of a flatsheet filter medium by sampling representative volumes of the upstream and downstream latex aerosol concentrations in a controlled airflow chamber. Filtered and dried air is passed through an atomizer to produce an aerosol containing suspended latex spheres. This aerosol is then passed through a charge neutralizer. The aerosol is then mixed and diluted with additional preconditioned air to produce a stable, neutralized, and dried aerosol of latex spheres to be used in the efficiency test. These test techniques can be used in filter medium testing for aerosol efficiency performance at discrete aerosol particle sizes for both manufacturers and users. It establishes a basis of efficiency comparison of one filter medium to another. For conventional filters, efficiencies less than about 30% on the average or for any particular filter is typically considered unacceptable since such a filter would pass a substantial proportion of the impinging particulate in the mobile fluid phase. Such an amount of particulate, in an engine application, in a gas turbine application or other such applications would pass substantially more particulate to the working parts of the mechanism such that substantial wear or failure of the mechanical device could result.
For the purpose of this patent application, the term xe2x80x9cmediaxe2x80x9d refers to a woven or non-woven sheet like substrate, having a thickness of about 0.1 to 5 millimeters and an efficiency of about 5% to 80% often 20% to 80%, made from a natural or synthetic fiber such as cellulose, polyester, nylon, polyolefin, etc.
For the purpose of this patent application, the term xe2x80x9cfine fiberxe2x80x9d refers to a fiber having a indeterminate length but a width of less than about 5 microns often, less than about 1 micron, formed into a randomly oriented mesh of fiber in a layer that substantially covers a surface of the media. We have found that there is a critical add-on amount of the fine fiber in this application. The fine fiber is placed onto opposing sides of a sheet like substrate an amount obtaining in a single layer of fine fiber an efficiency of about 15% to about 80%. Preferred add-on parameters are as follows:
In one embodiment, a reduced amount but useful add-on amount of fine fiber would be a 0.1 to 1.75 micron thick layer of 5% to 40% solidity fiber layer (95% to 60% void fraction). In this case the basis weight is 0.00045 to 0.11 mg-cmxe2x88x922 or 0.0028 to 0.7 lb.-3000 ftxe2x88x922 (lbs/3000 ftxe2x88x922 is a textile and paper makers standard unit).
In another embodiment, an add-on amount of fine fiber would a 0.75 to 1.25 micron thick layer of 15% to 25% solidity fiber layer (85% to 75% void fraction) In this case the basis weight is 0.010 to 0.05 mg-cm or 0.06 to 0.31 lb.-3000 ftxe2x88x922.
In a final embodiment, the upper end of the add-on amount of fine fiber would be a 1-3 micron thick layer of 10% to 40% solidity fiber layer (90% to 60% void fraction). In this case the basis weight is 0.009 to 0.2 mg-cmxe2x88x922 or 0.055 to 1.2 lb.-3000 ftxe2x88x922.
For the purpose of this patent application, the term xe2x80x9cseparate layerxe2x80x9d is defined to mean that in a filter structure having a substantially sheet-like substrate, that a fluid stream passing through the substrate must first pass through a first fine fiber layer, the substrate and subsequently pass through a second fine fiber layer. These layers can have a variety of filter geometry motifs. The fine fiber layers can in theory be manufactured in one processing step which covers the entirety of both surfaces of a two sided sheet-like substrate resulting in two separate fine fiber layers formed in the entirety on opposing sides of the substrate. In most applications, we envision that a first fine fiber layer will be formed on a substrate side and the substrate will then be passed again through the fine fiber generating step to form the second layer.
For the purpose of this patent application, the term xe2x80x9cfine fiber layer pore size or fine fiber web pore sizexe2x80x9d refers to a space formed between the intermingled fibers in the fine fiber layer.